Sequential sampling system

ABSTRACT

Although conventional sampling techniques give minimum risetime in the oscillography of repetitive electrical waveforms, system timing uncertainties introduce drift and jitter errors which are typically comparable in magnitude to the cathode ray oscilloscope risetime. By using two sampling oscilloscopes in cascade and in conjunction with special triggering methods, it is possible to reduce the drift by a factor of 10 3 down to a level of less than 10 14 sec./minute. The use of two sampling oscilloscopes in cascade thereby allows highly effective jitter filtering with the result that the new system has greatly improved accuracy in portraying the waveforms of very fast electrical phenomena.

United States Patent Elliott et al.

[54] SEQUENTIAL SAlVlPLlNG SYSTEM [72] Inventors: Brian J. Elliott, NewYork; John L.

Staples, Tarrytown, both of NY.

[73] Assignee: International Businoss Machines Corporation, Armonk, NY.

[22] Filed: July 13, 1970 [21] Appl. No.: 54,182

1451' Nov. 28, 1972 3,505,609 4/1970 versus at al. ..32s/1s1 x PrimaryExaminerRudolph V. Rolinec Assistant Examiner-Ernest F. KarlsenAttomey-Hanifin and Jancin and George Baron 1571 ABS'I'RACI Althoughconventional sampling techniques give minimum risetime in theoscillography of repetitive electrical waveforms, system timinguncertainties introduce drift and jitter errors which are typicallycomparable in magnitude to the cathode ray oscilloscope risetime. Byusing two sampling oscilloscopes in cascade and in conjunction withspecial triggering methods, it is possible to reduce the drift by afactor of 10* down to a level of less than 10 sec/minute. The use of twosampling oscilloscopes in cascade thereby allows highly effective jitterfiltering with the result that the new system has greatly improvedaccuracy in portraying the waveforms of very fast electrical phenomena.

3 Clains, 6 Drawing Figures 52 us. c1. ..324/121 R, 328/151 51 1m. 01...G01r 13/20, H031: 17/00 [58] Field 61 Search ..324/121; 328/151, 186;307/257; 315/25 [56] References Cited UNITED STATES PATENTS 3,011,12911/1961 Magleby et al. .307/257 x 3,229,212 1/1966 Rogers ..328/186 x3,248,655 4/1966 Kobbe et al ..328/1 86 x 1 3,493,875 2/1970 Stuckert..328/151 4 g PULSE SIGNAL GENERATOR y m CR0 TRIGGER LOW-PASS Vb(i)srcono vd 1 s/ummc 2? TRIGGER SWEEP Ve(i) FILTER Y 14 AUXILIARY x 111111511 RECORDER V111) 01 0111 A8 RATEIITEIIIIIII 2 GT2 31704.41 6

SNEU 1 BF 3 SIGNAL F|R3T Vb(r) SECOND VdIII PULSE SAMPLING SAMPLING mPASS GEIIERAToR CR0 CR0 FILTER TRIGGER TRIGGER SWEEP Ve(I) AUXILIARY Xwa TRIGGER RECORDER Vc(I) GIRcuIT N8 g a J o O F) l -n l I l U. I 50PIGG sEG Io,L SEC -|I T|ME INVENTORS BRIAN J. ELLIOTT FIG 2 JOHN L.STAPLES ATTORNEY PATENTEDnnvzs I912 sum 2.0F 3

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SHEET 3 0F 3 mun RANGE 100 SECONDS TIME F STOP RECORDER DRIFT-FREE mWAVEFORM 2 START 5 RECORDER O l- 10o SECONDS SEQUENTIAL SAMPLING SYSTEMBACKGROUND OF THE INVENTION Electronic sampling techniques are commonlyused in the oscillography of the repetitive waveforms of fast transientsbecause they have three advantages over conventional single-shotoscillographic methods: (1) improved time resolution and bandwidth, (2)greater linearity, and (3) greater sensitivity (at larger bandwidths).

When a,sampling oscilloscope is viewing the fastest waveforms (fromtunnel diode step generators), sweep speeds of picoseconds/cm aretypical. In this range, the timing errors in the sampling processes areconsiderable: even the best commercial circuits show time jitter errorsof about pico'se'conds and slow drift of similar magnitude. Jitter anddrift are, in practice, typically as great as the risetime of the systemand, therefore, they greatly limit the accuracy achieved by directviewing on the CRT-The jitter, being Gaussian in its probabilitydistribution and rapid, may be readily smoothed out by averaging, butthe drift, being highly irregular, slow and asymmetric, ,cannot beaveraged in practice. This prevents the use of averaging as a means ofnoise and jitter reduction.

To overcome the limitations noted above, a system has been devised forviewing constant repetition rate repetitive signals, whereby the analogoutput from one sampling oscilloscope is viewed with a second samplingoscilloscope, henceforth called Successive Sampling. A conventionalsampling oscilloscope views a repetitive input waveform but experiencestime drift of this waveform on the viewing screen. By using successivesampling, the second sampling oscilloscope can be synchronized to followthe drift of the output of the first so that the output of the secondremains stationary in its display. The tracking feature is achieved bytriggering the second sampling oscilloscope by means of an auxiliarytrigger circuit which senses the region of maximum slope of thewavefront of the smoothed output waveform of the first samplingoscilloscope. With the drift removed, the jitter may now be reduced bylowpass filtering, at the output of the second sampler, and the desiredwaveform is reproduced with the aid of an accurate recorder.

It is a primary object of this invention to employ two samplingoscilloscopes in cascade to reduce drift and jitter errors that normallyexist even with the best single sampling oscilloscope now available.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of the preferred embodiments of the invention as illustratedin the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of the system forachieving a substantially drift-free sampling of very fast risetimepulses.

FIGS. 26 are various input and output waveforms for aiding in theunderstanding of the operation of the system of FIG. 1.

The preferred embodiment of the invention shown in FIG. 1 includes afirst conventional sampling cathode ray oscilloscope 2 that isfree-running and triggering a pulse generator 4 that emits,periodically, a uniform pulse Va(t) to be analyzed. The main thrust ofthe invention to be described herein is to measure the detailed shape ofthe wavefront Va(t), shown in FIG. 2, with high accuracy.

The analog output signal waveform of sampling oscilloscope 2, which isthe same as the feedback loop signal from sampling oscilloscope 2 topulse generator 4, is Vb(t) as seen in FIG. 3. A low-pass filter 6smoothes Vb(t) to yield Vc(t), such smoothed wavefront Vc(t) (FIG. 4)serving to turn on auxiliary trigger circuit 8, the latter beingtriggered at a fixed level and generating a delayed step voltage totrigger a second conventional sampling oscilloscope 10. In this manner,a fixed relative reference time, T is always maintained with respect tothe mean wavefronts of Vc(t), and hence Vb(t), even as these wavefrontsdrift in time. In a manner to be described in greater detailhereinbelow, such sampling oscilloscope 10 takes a number of driftfreesamples from wavefront Vb(t) to produce a driftfree wavefront Vd(t). Thelatter may still have more time jitter than wavefront Vb(t), but suchoutput wavefront Vd(t) (FIG. 5) is smoothed by a second lowpass filter12 to'produce wavefront Ve(t) (FIG. 6), the latter being stored on asuitable recording device 14.

A better understanding of how the system of FIG. 1 operates can beobtained by considering waveforms depicted'in FIGS. 2 through 6 inconjunction with the system shown in FIG. 1.

In conventional single sampling oscilloscopes, a time controlled gate, apulse stretcher, a memory and a display are employed to yield a sloweddown replica of a real time waveform. For sampling oscilloscope 2, anywell-known unit can be used and models Hewlett- Packard 1430, 1425A,1411A and 141A are examples of types that were used for samplingoscilloscope 2. The second oscilloscope 10 should have a very high dotdensity capability as compared to that used for sampling oscilloscope 2.The actual one employed herein for oscilloscope 10 to carry out theinvention was a Tektronix S-2, 352,312, and a RM564.

When the timebase of the sampling oscilloscope 2 is free-running at apulse repetition frequency fl,, which is typically 10 pulses per second,and pulse generator 4 is a triggered tunnel diode, the latter emits apulse whose input waveform is Va(t) as shown in FIG. 2. The pur pose ofthe successive sampling system forming the present invention is tomeasure the detailed shape of the 50 picosecond pulse (from a to b) withhigh accuracy. As seen in FIG. 2, repetitive pulses to be measured recurevery 10 microseconds (T) as the pulse generator 4 is triggered bysampling oscilloscope 2.

The analog output of sampling oscilloscope 2 is shown in FIG. 3 aswaveform Vb(t). FIG. 3 is a slowed down replica of the desired wavefront(from a to b of pling oscilloscope is used to reproduce a waveformVb(r).

The low-pass filter 6 serves to smooth out waveform Vb(t) of FIG. 3 byaveraging out the jitter to produce the smoother waveform Vc(t) shown inFIG. 4. Some residual jitter-induced noise remains, as shown in FIG. 4,but its effect is negligible. This smoother waveform drives theauxiliary trigger circuit 8. The latter is always triggered at apredetermined voltage level V,,. Hence a fixed relative reference time Tis determined for each wavefront of Vb(t). Then the trigger circuit 8generates a delayed signal to trigger the second sampling oscilloscopeIt). in this manner the second oscilloscope 10 is slaved to follow thedrift of the output of the first sampling oscilloscope 2 so that theoutput wavefront of the second oscilloscope remains stationary. Thisrelative non-movement is achieved in the following way. The secondsampling oscilloscope 10 is triggered by means of the auxiliary triggercircuit 8 which senses and thereby automatically corrects for therelative slow motion (drift) of the output waveform Vb(t) of the firstsampling oscilloscope 2. It is important that the relative timepositions of the repetitive wavefronts Vc(t) exactly follow the meanpositions of the noisier wavefronts Vb(t).

This is so because the drifting sampled signal Vb(t) is fed into thelow-pass filter 6 to produce Vc(t). The purpose of the low-pass filter 6is simply to reduce the amount of time jitter that is transferred toVdit) (See FIG. when the second sampling oscilloscope is triggered. inthis way, the noisy, but now drift-free, output Vd(t) has lessjitter-induced noise and therefore requires less filtering by low-passfilter 12. This in turn reduces the amount of time needed to accuratelyaverage and record the highly smoothed output waveform Ve(t) shown inFIG. 6. With the drift removed, the jitter may now be reduced bylow-pass filtering, at the output of the second oscilloscope and thedesired waveform can be accurately reproduced with the aid of a recorderor a computer.

The invention, as shown and described above, has reduced drift by afactor of 10' so that absolute time calibration is possible with a 3X10'sec. resetting capability and fast waveforms may be accurately recordedwith a time measurement uncertainty of 10' sec.

What is claimed is: I

l. A sequential sampling system for eliminating the effects of theinherent drift of single sampling systems, comprising:

a cascaded pair of sampling oscilloscopes wherein the repetitive analogoutput signal of the first sampling oscilloscope, produced by sampling arepetitive input signal thereto, is coupled to a second samplingoscilloscope,

a triggering means having the output thereof coupled to said secondsampling oscilloscope for controlling the sampling time of said secondsampling oscilloscope in accordance with the trigger pulses therefromresponsive to be initiated by the said repetitive output signal of saidfirst sampling oscilloscope so as to thereby avoid the effects due todrift in said first oscilloscope,

a low-pass filter means coupling the said repetitive 5 output signal ofsaid first sampling oscilloscope to said triggering means so as tothereby diminish the time-jitter that would normall be transmittedthrough said trigger circuit to sm second sampling oscilloscope, andcause said triggering means to sense in time, a point of fixed height ona region substantially at the maximum slope of the respective repetitivewaveforms of said repetitive output signal of the firstsamplingoscilloscope to thereby produce said trigger pulses so that thesecond sampling oscilloscope is thereby synchronized to follow the driftintroduced by the first sampling oscilloscope whereby the output signalof said second sampling oscilloscope is displayed as a stationarywaveform.

2. The system of claim 1 wherein means including recording means areconnected to said second sampling oscilloscope for recording thedrift-free, sloweddown replica of said repetitive input signal, saidmeans further including a low-pass filter interconnected between theoutput signal of said second sampling oscilloscope and said recordingmeans so that the timejitter of said output waveform is effectivelyremoved without distortion.

3. A sequential sampling system for reducing the drift normallyoccurring in a single sampling oscilloscope, comprising:

signal producing means for producing a repetitive signal;

a first sampling oscilloscope means coupled to said signal producingmeans for sampling said repetitive signal therefrom and providing areproduced repetitive signal at the output thereof;

trigger-circuit means coupled to the said output of said first samplingoscilloscope means and responsive to be triggered by said reproducedrepetitive signal therefrom to produce an output pulse each time saidreproduced signal reaches a predetermined voltage level;

second sampling oscilloscope means coupled to the said output of saidfirst sampling oscilloscope means and to said trigger-circuit means tosample said reproduced repetitive signal from the said output of saidfirst sampling oscilloscope means each time a pulse is produced by saidtrigger-circuit means so as to thereby cause said second samplingoscilloscope means to be synchronized to follow the drift introducedinto said reproduced repetitive signal by said first samplingoscilloscope means, whereby the output signal of said second samplingoscilloscope means is stationary.

1. A sequential sampling system for eliminating the effects of theinherent drift of single sampling systems, comprising: a cascaded pairof sampling oscilloscopes wherein the repetitive analog output signal ofthe first sampling oscilloscope, produced by sampling a repetitive inputsignal thereto, is coupled to a second sampling oscilloscope, atriggering means having the output thereof coupled to said secondsampling oscilloscope for controlling the sampling time of said secondsampling oscilloscope in accordance with the trigger pulses therefromresponsive to be initiated by the said repetitive output signal of saidfirst sampling oscilloscope so as to thereby avoid the effects due todrift in said first oscilloscope, a low-pass filter means coupling thesaid repetitive output signal of said first sampling oscilloscope tosaid triggering means so as to thereby diminish the time-jitter thatwould normally be transmitted through said trigger circuit to saidsecond sampling oscilloscope, and cause said triggering means to sensein time, a point of fixed height on a region substantiallY at themaximum slope of the respective repetitive waveforms of said repetitiveoutput signal of the first sampling oscilloscope to thereby produce saidtrigger pulses so that the second sampling oscilloscope is therebysynchronized to follow the drift introduced by the first samplingoscilloscope whereby the output signal of said second samplingoscilloscope is displayed as a stationary waveform.
 2. The system ofclaim 1 wherein means including recording means are connected to saidsecond sampling oscilloscope for recording the drift-free, slowed-downreplica of said repetitive input signal, said means further including alow-pass filter interconnected between the output signal of said secondsampling oscilloscope and said recording means so that the time-jitterof said output waveform is effectively removed without distortion.
 3. Asequential sampling system for reducing the drift normally occurring ina single sampling oscilloscope, comprising: signal producing means forproducing a repetitive signal; a first sampling oscilloscope meanscoupled to said signal producing means for sampling said repetitivesignal therefrom and providing a reproduced repetitive signal at theoutput thereof; trigger-circuit means coupled to the said output of saidfirst sampling oscilloscope means and responsive to be triggered by saidreproduced repetitive signal therefrom to produce an output pulse eachtime said reproduced signal reaches a predetermined voltage level;second sampling oscilloscope means coupled to the said output of saidfirst sampling oscilloscope means and to said trigger-circuit means tosample said reproduced repetitive signal from the said output of saidfirst sampling oscilloscope means each time a pulse is produced by saidtrigger-circuit means so as to thereby cause said second samplingoscilloscope means to be synchronized to follow the drift introducedinto said reproduced repetitive signal by said first samplingoscilloscope means, whereby the output signal of said second samplingoscilloscope means is stationary.